(1) Field of the Invention
The present invention relates to a method of controlling a group of engines driving a rotor, such as a rotor for providing a rotorcraft with lift and possibly also propulsion. Furthermore, the invention relates to an aircraft implementing the method.
Under such circumstances, the invention lies in the technical field of groups of aircraft engines, and particularly groups of engines for rotary wing aircraft, such a group comprising a plurality of engines and at least one electric motor.
(2) Description of Related Art
Conventionally, a rotary wing aircraft includes in principle a group of engines comprising at least one fuel-burning engine such as a piston engine or a turbine engine. A gearbox connects the group of engines to the main advance and lift rotor: this is referred to as the main gearbox (MGB).
Temperature limits for an engine and torque limits for a main gearbox serve to define an operating envelope for each engine that covers three normal utilization ratings:
a takeoff rating corresponding to a level of torque for the main gearbox and a level of heating for the engine that can be accepted for a limited length of time without significant degradation, this takeoff rating being defined by a maximum takeoff power (max TOP) and by a duration for using this maximum takeoff power that is generally of the order of five minutes;
a maximum continuous power (MCP) rating that corresponds to about 90% of the maximum takeoff power max TOP and to a duration of utilization of this maximum continuous power that is generally unlimited; and
an idling rating for minimizing fuel consumption, with the engine nevertheless continuing to keep running while idling.
By way of example the idling rating may be used to keep the speed at which a moving member of the engine moves at a given value.
For example, the speed of rotation of a gas generator may be maintained for a turbine engine. The specific value is set by the manufacturer, in particular for optimizing the fuel consumption of the engine and for ensuring that the engine continues to run independently.
As a variant, for a turbine engine having a free turbine, it is possible to control the speed of rotation of the free turbine.
Furthermore, an aircraft and in particular a rotary wing aircraft may have one or more engines. For example, three categories of rotorcraft may be distinguished.
The first category relates to single-engine rotorcraft where there is only one engine, a piston engine or a turbine engine. In the absence of any other source of power, it is not possible to share the supply of power between different sources.
The second category relates to twin-engined rotorcraft where two engines are provided, two piston engines or two turbine engines. Those engines are controlled jointly so that each supplies half of the power required at any moment in flight, with this required power being referred to below as the “necessary” power.
Alternatively, the two engines may be controlled so that some of their operating parameters are kept equal, such as for example the speed of rotation of a gas generator or the control temperature in the context of fuel-burning engines.
Regulating engines in that way does not enable them to operate in asymmetrical manner, except in the event of one of them failing. In particular, deliberately stopping one of the engines in flight or causing it to idle is prohibited for safety reasons.
A third category relates to rotorcraft having three engines and they are similar in terms of operation to twin-engine aircraft.
Thus, on a rotorcraft having a plurality of engines, the trend is to share the amount of power that needs to be developed fairly between the various engines.
However, sharing power in that way can lead to engines being used in operating ranges that have low energy efficiency.
It should be observed that the specific fuel consumption of a turbine engine drops with increasing power developed by that engine up to an optimum point referred to as the “adaptation point”, which is generally close to the maximum takeoff power max TOP. Surprisingly, the greater the level of power developed by a turbine engine, the better its specific consumption, up to close to the maximum takeoff power max TOP.
Under such circumstances, sharing the necessary power in equivalent manner between a plurality of engines in a group of engines tends to cause all of the engines to operate in operating ranges that are not optimized from the point of view of energy efficiency.
The duration of a flight or the distance that can be traveled by the aircraft are thus reduced.
A rotorcraft has two characteristic forward speeds:
a first speed is known as “velocity of best endurance” (Vbe) and it corresponds to the horizontal speed that provides the rotorcraft with maximum endurance to enable it to fly for as long as possible with a given quantity of fuel; and
a second speed known as the “velocity of best range” (Vbr) corresponding to the horizontal speed that provides the rotorcraft with a maximum distance that it can travel in order to enable it to fly as far as possible with a given quantity of fuel.
Nevertheless, it is found for example that the first speed can generally be achieved by a rotorcraft having two or three engines by using a single engine delivering power that is less than or equal to the maximum continuous power MCP.
It can thus be understood that when flying a rotorcraft at this first speed while running all of the engines, each engine is called on to develop a relatively moderate level of power, thereby giving rise to medium energy efficiency.
Under such circumstances, it is possible to envisage stopping one of the engines in flight in order to improve energy efficiency. Nevertheless, although stopping an engine is possible, it should be observed that the flight envelope is then generally very restricted.
For example, it is observed that it is often difficult or even impossible to fly at the second speed using only one engine on a twin-engined rotorcraft.
Consequently, performing long-range missions on one engine requires the forward speed to be reduced. Under such circumstances, the saving in fuel consumption achieved by optimizing use of the turbine engine can be countered by a reduction in the energy performance of the rotorcraft, since the rotorcraft is no longer operating at the second speed that is optimum for this type of mission. Furthermore, travel time may be lengthened significantly.
Under such circumstances, a flight made to cover a long distance must advantageously be performed while using all of the engines, even though that means using them in an operating range that is not optimized from an energy efficiency point of view.
It should be observed that the prior art includes document FR 2 914 697 relating to a device for providing assistance during transient stages in which an aircraft is accelerating or decelerating.
That document FR 2 914 697 describes a turbine engine having a gas generator, a free turbine driven in rotation by the stream of gas generated by the gas generator, and an auxiliary motor.
In order to avoid a known “pumping” phenomenon, an engine manufacturer generally provides a so-called “pumping” margin that limits the acceleration capacity of the turbine engine. That document FR 2 914 697 relates to a helicopter turbine engine presenting acceleration capacity that is optimized, while still having the same pumping margin as a prior art turbine engine. The turbine engine then includes an auxiliary motor coupled to a shaft of the gas generator in order to supply an additional quantity of rotary kinetic energy to the shaft during a stage in which the turbine engine is accelerating.
Document FR 2 933 910 describes a hybrid installation having at least one engine and at least one electric motor.
Document DE 10 2007 017332 describes an aircraft having a propeller, an internal combustion engine having a drive take-off enabling the propeller to be driven, and an electrical machine, the internal combustion engine co-operating with the electrical machine.
The invention thus provides a method of controlling a group of engines of an aircraft developing a necessary power for driving a rotor, the group of engines being provided with at least one electrical member connected to electrical energy storage means and with a first number n of fuel-burning engines greater than or equal to two. The electrical member may then comprise at least one electric motor capable of operating in an electric motor mode and in an electricity generator mode, the storage means possibly comprising at least one battery.